Thermal transpiration or thermal creep refers to the thermal force on a gas due to a temperature difference. Thermal transpiration generates a flow of gas in the absence of pressure differences, and maintains a certain pressure difference in a steady state. In most applications, the effect is strongest when the mean free path of the gas molecules is comparable to the dimensions of a container or device.
A well-known device which relies on thermal transpiration is Crookes' Radiometer, also known as a light mill. Generally the light mill is a small chamber containing typically four or more vanes mounted symmetrically around a vertically-oriented axle, with opposing sides of each vane generally parallel to the axle. The parallel sides of the vanes are configured as a high emissivity surface on one side and a lower emissivity surface on an opposite side. When intense light impinges on the vessel, the temperature of the higher emissivity side becomes greater than the lower emissivity side, and the temperature difference generates a force directed toward the colder surface as air molecules contained in the vessel strike on the vanes. See e.g., Scandurra et al., “Gas kinetic forces on thin plates in the presence of thermal gradients,” Physical Review E 75(2) (2007), among others. At low pressure the exerted forces are generally proportional to the temperature gradient on the vane, as well as the mean free path of gas molecules, the density of the gas, cross section of the molecules, and other factors, and the exact mechanism is a complex problem of kinetic theory of gases. In light mills where the differing emissivity surfaces occupy opposing sides of the vane, a thermal flow of molecules occurs from the cold to the hot side of the vane, and the reaction to this streaming is a force directed opposite to the temperature gradient, in a direction generally normal to the hotter surface. The maximum effective thickness of this force is one the order of a mean free path length of the surrounding gas, and correspondingly light mills are typically constrained to rarified atmospheres. See, for example, U.S. Pat. No. 4,410,805 issued to Berley, issued Oct. 18, 1983, and U.S. Pat. No. 4,397,150 issued to Paller, issued Aug. 9, 1983, among others. Additionally, in light mills where the responsible surfaces are essentially normal to a direction of intended rotation, the light absorbing surfaces present themselves as significant blunt drag objects in the surrounding atmosphere, which tends to impede motion as surrounding pressure is increased.
Thermal transpiration has also been employed to address challenges inherent to miniaturized moving parts, such as micropumps. See e.g. U.S. Pat. No. 5,871,336 issued to Young, issued Feb. 16, 1999, and see U.S. Pat. No. 8,235,675 issued to Gianchandani et al., issued Aug. 7, 2012, and see U.S. Pat. No. 5,611,208 issued to Hemmerich et al., issued Mar. 18, 1997, among others. In these applications, thermal transpiration is employed in a narrow channel to sustain a non-zero longitudinal pressure gradient when subjected to a temperature bias, where the narrow channel has hydraulic diameter smaller than the mean free path of the gas molecules and the temperature gradient is generally parallel to the confining walls of the channel. One accepted physical mechanism which explains the phenomenon posits that an asymmetric momentum transfer between the gas molecules and the channel walls is primarily responsible, since gas molecules from hot areas have a higher average velocity compared to the molecules from the cold side. In Knudsen-type pumps, this asymmetry results in an effective momentum transfer to the channel walls in the direction opposite to the temperature gradient. Although the wall is stationary, this does generate a force parallel to the channel surface, as opposed to generating forces normal to the surface as occurs in the light mills employing vertically mounted vanes.
It would be advantageous to provide a light activated rotor which generates thermal creep and corresponding momentum transfers parallel to a vane surface in order to generate rotary motion in response to a radiant flux such as light. Such a light activated rotor would be free of constraints closely tying vane thickness to mean free path lengths, and further would greatly mitigate the impact of light absorbing surfaces acting as blunt force drag objects tending to impede the generated rotary motion. Additionally, such as device would be more amenable to miniaturization allowing potential operation at normal atmospheric pressures.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.